1. Field of the Invention
The present invention is related to the field of well drilling. In particular, the present invention provides an improved method and apparatus for drilling highly deviated and horizontal wells in a hydrocarbon bearing formation.
2. Description of Related Art
In the field of petroleum exploration and production, the use of directionally drilled wells is commonplace. Directionally drilled wells are commonly used in offshore locations, urban locations, and other locations where it becomes necessary to drill wells into formations that are laterally displaced from the drilling location.
With recent downturns in petroleum prices, it has become increasingly important from an economic point of view to more fully develop known petroleum reserves rather than explore for new reserves. Highly deviated, or horizontally drilled wells have, therefore, been increasingly utilized to develop those known petroleum reserves. Horizontal wells can be used, for example, to contact vertical fractures and other fracture systems which would otherwise be inaccessible from vertical or nearly vertical wells.
A variety of methods have been proposed for drilling highly deviated or horizontal wells. For example, the use of down-hole motors is known and has been used in both the petroleum business and in under-river drilling, coal drilling, and the like. The use of down-hole motors using a bent motor housing is disclosed in, for example, Rehm, "Horizontal Drilling Applied in Slim Holes," Petroleum Engineer International, Feb. 1987. See also U.S. Pat. No. 4,789,032, entitled "Orienting an Circulating Sub", and Rehm, "Horizontal Drilling In Mature Oil Fields," SPE Paper No. 18709 (1989), both of which are incorporated herein by reference for all purposes.
In the above-described horizontal drilling methods, a vertical section of well is drilled using methods well known to those of skill in the art. Alternatively, an existing vertical or nearly vertical well may be utilized. A section of casing is milled out and a cement plug is placed in the milled-out section. After drilling out a short distance (e.g., 20 feet), a sidecutting or "turning" drilling assembly is run into the hole. The side-cutting drilling assembly generally includes a down-hole motor which rotates an adjacent bit.
The down-hole motor is utilized to drill a curved portion of the well. The motor is driven by drilling fluids pumped from the surface and is housed in a "bent" housing. The bent housing provides the necessary force to drill the well in a curved fashion. Curvature is, alternatively, induced by a bent sub to which the motor is mounted and/or a pad mounted to the drill string adjacent the sub.
In order to drill the well in the desired direction, a mule key orienting sub is provided. A suitable mule key orienting sub is disclosed in U.S. Pat. No. 4,789,032. Drilling of the well is monitored with survey equipment of the type known to those of skill in the art. Upon reaching a desired drilling direction (e.g., horizontal) the bent drilling assembly is replaced with a straight drilling assembly, and drilling continues.
In using the prior known methods of drilling highly deviated wells, a number of problems have arisen. In particular, it has been found that drill bits (which generally have a long gauge shoulder) have been of limited value in that their drilling rate is slow, and excessive wear on the gauge and center has occurred.
Further, it has been observed that the downhole motor creates significant vibration (due at least in part to the high motor speeds of 600--700 RPM) which can lead to failure cf the steering mechanism and that in some cases the motor lifts off of the bottom side of the hole. This factor, along with others, can also result in curvature of the wellbore that is outside of the range that would be predicted for the drilling assembly.
Still further, in conventional turning assemblies, pick-up drag can be up to 50% of the free weight of the drilling assembly. This can cause particular problems in well recompletion operations wherein it is often desirable to use smaller drilling rigs.
Finally, when turning to horizontal, drag forces on the drill pipe occur which make it difficult to control the weight on the bit. Accordingly, it is desirable to have a down-hole assembly that turns the curve independent of weight. Conventional directional drilling assemblies use weight (on the bit) as a method of forcing the assembly to turn. They turn typically at 1/10 of the rate of this assembly and are unable to consistently turn at rates above six degrees per 100 feet.
Accordingly, it is seen that an improved method and apparatus for drilling deviated wells is desired.